Generally, it is old to provide a subsurface well safety valve for use in a well for shutting off flow of well fluids through the well tubing. U.S. Pat. No. 3,782,461 discloses a safety valve in which the valve is opened by a piston in response to hydraulic fluid applied from the well surface and is biased to a closed position by suitable means including a pressurized gas chamber acting on the piston. Generally, the means biasing the valve to a closed position must overcome the hydrostatic head in the hydraulic control line to the piston as well as providing a closing force. Because the hydrostatic forces increase with depth, the gas in the chamber must be increasingly pressurized in order to utilize the safety valve at greater depths. However, there are limits to which the pressure in the gas chamber in a safety valve may be increased. Furthermore, when a conventional piston actuated safety valve is open, the piston acts against the biasing gas in the pressurized chamber to further increase the pressure in the gas chamber. Thus, there is a differential in the pressure in the gas chamber or "spread" between opening and closing pressures which limits the closing pressure that can be applied to a safety valve and in turn limits the depth at which the safety valve can be set without exceeding the pressure limitations in the gas chamber. For example, the differential or spread between the opening and closing pressure in a typical safety valve such as shown in U.S. Pat. No. 3,782,461 may be 1500 psi. If the differential pressure or spread is reduced, such as to 100 pounds or less, the safety valve may be utilized with a higher closing pressure and thus set at greater depths.
The present invention is directed to various improvements in a piston actuated subsurface well safety valve which is biased to a closed position by a pressurized gas chamber in which a structure is provided that reduces the cross-sectional area of the piston which in turn reduces the pressure buildup in the gas chamber when the valve is opened thereby reducing the differential between the opening and closing forces to allow the valve to be set at greater depths as well as reducing undesired seal areas in the valve.